U.S. Pat. No. 4,170,821 issued Oct. 16, 1979 describes a razor cartridge having a cap with a lubricating composition. It also describes the incorporation of this agent in a water insoluble microporous substrate.
Commercially available razors of this type namely Gillette ATRA PLUS and Schick ULTREX PLUS provide the combination of a lubricating strip which is subsequently affixed to the razor cap. This strip, containing the shaving aid, which is polyethylene oxide having a molecular weight between 100,000 and 6,000,000, is released from a microporous substrate, i.e., typically polystyrene by leaching.
The process for manufacturing a razor having such a lubricating strip employs first, injection molding of the cap and then the separate attachment of the strip. The strip is attached either by the use of acrylate adhesives or by mechanical means. When an adhesive is utilized, the combination of the strip and cap must be properly positioned after the adhesive is applied and then clamped for a period of time to allow the initial adhesive bonding to occur. This process has the disadvantage of the extra cost associated with the use of the adhesive as well as the separate steps utilized to mate and cure the adhesive.
A mechanical attachment means usually involves a slot defining a recess in the top surface of the cap generally extending longitudinally along the cap length and a positioning means either at the end of the recess or at the bottom of the recess or in both places. The strip which is separately manufactured and which is either extruded or injection molded is cut, positioned and retained usually by means of tabs or the like which can be bent over a portion of the strip to retain it.
Ideally, the steps relating to the marriage of the separately formed strip adn cap would be avoided if the strip could be molded in the same machine after molding of the cap had been completed. While the mixture of polyethylene oxide and polystyrene can be rendered plastic and deformable, attempts to sequentially mold the polystyrene polyethylene combination have run into some substantial difficulties.
One of the problems inherent in attempting to injection mold of a polyethylene oxide compound is that high molecular weight polyethylene oxides are preferred for this particular application because they have the desired rate of water solubility. Lower molecular weight polyethylene oxide compounds, i.e., those near the bottom of the range disclosed in the above mentioned Booth patent, tend to rapidly leach out of the polystyrene open-celled matrix or honeycomb structure and may be essentially "used up" before the number of shaves contemplated by the particular blade assembly is completed.
The desired, high molecular weight polyethylene oxide is, unfortunately, highly susceptible to chain scission which reduces its molecular weight and consequently its efficacy as a shaving lid. In the thermoplastic state high molecular weight polyethylene oxide has an extremely high melt viscosity. Therefore, in order to sequentially mold the lubricating strip onto the razor cap, it is necessary to use very high injection molding temperatures to achieve the necessary melt flow to successfully complete the injection molding of the strip. This combination of high temperature and shear exposure accelerates the degradation of the polyethylene oxide via chain scisson. This problem could be substantially reduced if the temperatures used for injection molding were substantially reduced.
Another problem associated with the use of very high injection molding temperatures in the sequential molding process is the potential thermal distortion of the previously molded cap during the sequential molding of the lubricating strip.
For these reasons, and the obvious energy savings, it is highly desirable to be able to substantially reduce the injection molding temperatures used to form the lubricating strip.